c# - ModelState.isValid - 错误

Question

我想从 html 页面上的 dropdownList 获取参数并将其发送到我的 Controller ，创建新的模型对象，并将其插入数据库。

这是我的 Controller (创建 My_Model 的两种方法):

public ActionResult Create()
    {
        IEnumerable<MusicStyle> musicStyleList = db.MusicStyles.ToList();
        ViewData["musicStyles"] = new SelectList(musicStyleList);
        return View();
    }

    // POST: Bands/Create
    // To protect from overposting attacks, please enable the specific properties you want to bind to, for 
    // more details see http://go.microsoft.com/fwlink/?LinkId=317598.
    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "id,name,info,appearanceDate,musicStyles")] Band band)
    {
        IEnumerable<MusicStyle> musicStyleList;
        if (ModelState.IsValid)
        {
            musicStyleList = db.MusicStyles;
            ViewData["musicStyles"] = new SelectList(musicStyleList).ToList();
            db.Bands.Add(band);
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        musicStyleList = db.MusicStyles;
        ViewData["musicStyles"] = new SelectList(musicStyleList).ToList();
        return View(band);
    }

这是我在 html 页面上的下拉列表:

@Html.DropDownList("musicStyles", "select style")

这是我的模型:

 public class Band
{
    [Required]
    public int id { get; set; }

    [Required]
    [RegularExpression(@"^[a-zA-Z-\s\\\/*_]+$")]
    [StringLength(60, MinimumLength = 1)]
    public string name { get; set; } 

    public string info { get; set; }

    [Required]
    [Display(Name = "Appearance date")]
    [DataType(DataType.Date)]
    [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}")]
    public DateTime appearanceDate { get; set; }

    public List<MusicStyle> musicStyles { get; set; }
    public List<Song> songs { get; set; }
    public List<Album> albums { get; set; }
}

它有一个 musicStyles 列表(引用多对多)，我想将 dropdownList 中的选定元素设置为该模型。但是在 Create 方法的结果中，我有一个 null musicStyles 和 ModelState.isValid == false

Answer 1

A <select>仅回发一个值 - 它无法绑定(bind)到属性 List<MusicStyle> musicStyles您需要一个具有可以绑定(bind)到的属性的 View 模型

public class BandVM
{
  [Display(Name = "Music style")]
  [Required(ErrorMessage = "Please select a style")]
  public string SelectedMusicStyle { get; set; }
  public SelectList MusicStyleList { get; set; }
  ....
}

在 Controller 中

public ActionResult Create()
{
  BandVM model = new BandVM();
  model.MusicStyleList = new SelectList(db.MusicStyles);
  return View(model);
}

在 View 中

@Html.LabelFor(m => m.SelectedMusicStyle)
@Html.DropDownListFor(m => m.SelectedMusicStyle, Model.MusicStyleList, "select style")
@Html.ValidationMessageFor(m => m.SelectedMusicStyle)

在 POST 方法中

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(BandVM model) // NO Bind.Include!
{
  // model.SelectedMusicStyle will contain the selected value
  // Create a new instance of the data model and map the view model properties to it
  // Save and redirect
}